Downhole stabilisers are tools which are coupled into the drill string to bear against the wall of the drilled hole and thus centralise the drill string in the hole. Such stabilisers often have several spiral fin-like blades externally studded with wear-resistant inserts. The gaps between the stabiliser blades allow relatively free passage of drilling mud and fine debris. Although necessary for the stability of drill strings, conventional downhole stabilisers have the disadvantage of presenting frictional resistance to rotation of the drill string in the drilled hole, and frictional resistance to raising and lowering of the drill string in the hole.
A downhole tool not unlike a stabiliser is a device known as a reamer, which serves to smooth and enlarge the drilled hole by means of peripheral cutters carried on the reamer. Some types of reamer are known in which the cutters are in the form of peripherally-mounted rollers which rotate on axes parallel to but offset from the central axis of the drill string. Apart from torque specifically required for reaming, it may be expected that the turning resistance of a roller reamer is lower than for a non-rolling reamer, but undiminished for lifting and lowering. Thus, even if a stabiliser were fitted with rollers to diminish turning friction, friction incurred during lifting and lowering would not be diminished.
It is therefore an object of the present invention to provide a downhole stabiliser in which such problems of friction are obviated or mitigated.